The present invention generally pertains to code sequence tracking in Code Division Multiple Access (CDMA) communication systems, also known as spread-spectrum communication systems. More particularly, the present invention pertains to a system and method for efficient tracking of multipath signal components allowing for combining of multipath signal components to improve data signal detection and despreading by reducing effects of multipath interference, and increase CDMA communication system efficiency by reducing the required transmit power.
Providing quality telecommunication services to user groups which are classified as remote, such as rural telephone systems and telephone systems in underdeveloped countries, has proved to be a challenge over recent years. The past needs created by these services have been partially satisfied by wireless radio services, such as fixed or mobile frequency division multiplex (FDM), frequency division multiple access (FDMA), time division multiplex (TDM), time division multiple access (TDMA) systems, combination frequency and time division systems (FD/TDMA), and other land mobile radio systems. Usually, these remote services are faced with more potential users than can be supported simultaneously by their frequency or spectral bandwidth capacity.
Recognizing these limitations, recent advances in wireless communications have used spread spectrum modulation techniques to provide simultaneous communication by multiple users. Spread spectrum modulation refers to modulating a information signal with a spreading code signal; the spreading code signal being generated by a code generator where the period, Tc, of the spreading code is substantially less than the period of the information data bit or symbol signal. The code may modulate the carrier frequency upon which the information has been sent, called frequency-hopped spreading, or may directly modulate the signal by multiplying the spreading code with the information data signal, called direct-sequence spreading (DS). Spread-spectrum modulation produces a signal with bandwidth substantially greater than that required to transmit the information signal. The original information is recovered by synchronously demodulating and despreading of the signal at the receiver. The synchronous demodulator uses a reference signal to synchronize the despreading circuits to the input spread-spectrum modulated signal to recover the carrier and information signals. The reference signal may be a spreading code which is not modulated by an information signal. Such use of a synchronous spread-spectrum modulation and demodulation for wireless communication is described in U.S. Pat. No. 5,228,056 entitled SYNCHRONOUS SPREAD-SPECTRUM COMMUNICATIONS SYSTEM AND METHOD by Donald L. Schilling, which is incorporated herein by reference.
One area in which spread-spectrum techniques are used is in the field of mobile cellular communications to provide personal communication services (PCS). Such systems desirably support large numbers of users, control Doppler shift and fade, and provide high speed digital data signals with low bit error rates. These systems employ a family of orthogonal or quasi-orthogonal spreading codes, with a pilot spreading code sequence that is synchronized to the family of codes. Each user is assigned one of the spreading codes from the family as a spreading function. Related problems of such a system include handling multipath fading effects. Solutions to such problems include diversity combining of multipath signals. The problems associated with spread spectrum communications, and methods to increase capacity of a multiple access, spread-spectrum system are described in U.S. Pat. No. 4,901,307 entitled SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS by Gilhousen et al. which is incorporated herein by reference.
The problems associated with the prior art systems focus around reliable reception and synchronization of the receiver despreading circuits to the received signal. The presence of multipath fading introduces a particular problem with spread spectrum receivers in that a receiver must somehow track the multipath components to maintain code-phase lock of the receiver""s despreading means with the input signal. Prior art receivers generally track only one or two of the multipath signals, but this method may not be satisfactory because the combined group of low-power multipath signal components may actually contain far more power than the one or two strongest multipath components. The prior art receivers track and combine only the strongest components to maintain a predetermined Bit Error Rate (BER) of the receiver. Such a receiver is described, for example, in U.S. Pat. No. 5,109,390 entitled DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM by Gilhousen et al. which is incorporated herein by reference. A receiver that combines all multipath components, however, is able to maintain the desired BER with a signal power that is lower than that of prior art systems because more signal power is available to the receiver. Consequently, there is a need for a spread spectrum communication system employing a receiver that tracks substantially all of the multipath signal components, so that substantially all multipath signals may be combined in the receiver. This would reduce the required transmit power of the signal for a given BER.
The present invention is embodied in a multiple access, spread-spectrum communication tracking system which processes a plurality of multipath signal components of a code-division-multiplexed (CDM) signal received over a radio frequency (RF) channel. The system and method tracks a centroid of a group of multipath spread-spectrum signal components constituting a spread-spectrum channel signal which includes a transmitted code sequence. The exemplary system and method operate by digitally sampling the spread-spectrum channel signal to produce a sequence of sample values. The sample values are divided into a set of even-numbered sample values which define a sequence of early spread-spectrum channel signal samples corresponding to the early multipath signal components and a set of odd sample number values which define a sequence of late spread-spectrum channel signal samples corresponding to the late multipath signal components
The centroid tracking receiver generates a plurality of local code sequences, each of which has a code phase and symbol period, and each of which is a code phase-shifted version of the transmitted code sequence. The centroid tracking receiver correlates each of the locally generated code sequences with the sequence of early received spread-spectrum channel signal samples to produce a group of early despread multipath signals. The tracking receiver also correlates each of the locally generated code sequences with the sequence of late received spread-spectrum channel signal samples to produce a group of late despread multipath signals. The group of early despread multipath signals are weighted according; to a predetermined algorithm and processed to produce an early tracking value, and the group of late despread multipath signals are similarly weighted and processed to produce a late tracking value.
The difference between the early tracking value and the late tracking value is calculated to produce an error signal value. Finally, the centroid tracking system adjusts the code phase of each of the locally generated code sequences responsive to the error signal value to maintain the maximum received signal energy.